Suggested applications of the recently discovered superconducting compounds, including superconducting ceramics composed of Ba--Y--Cu--O, include microelectronics. In such applications, thin films of the superconductor would provide electronic circuits. Prior efforts to provide a Ba--Y--Cu--O thin film superconductor include deposition thereof, by various means, onto crystalline substrates such as Si, GaAs, MgO, Al.sub.2 O.sub.3, BaTiO.sub.3, (BaSr)TiO.sub.3, SrTiO.sub.3, ZrO.sub.2, glass and others. Although these deposited thin films were in some cases superconducting at liquid nitrogen temperature, none were truly epitaxial; all were polycrystalline. Depending on the method of preparation, the grain size in these polycrystalline films may vary from a few hundredth of a micron to several micron. The reason for polycrystalline film formation on these substrates is found in lattice mismatch between substrate and film. Among the substrates reported so far, SrTiO.sub.3 has the closest lattice match (a=b=3.905 .ANG.) to Ba.sub.2 YCu.sub.3 O.sub.7-x (a=3.824 .ANG., b=3.884 .ANG.). Indeed, film grown on SrTiO.sub.3 had the best orientation and showed the highest critical current density of 10.sup.5 A/cm.sup.2 at 77K, and 10.sup.6 A/cm.sup.2 at 4.2K (Oh et al., Appl. Phys. Lett. 51, 852 (1987)).
Grain boundaries in thin film superconductors are undesirable because they limit the performance of the material and introduce processing difficulties of making fine circuit patterns.
It is an object of the present invention to provide crystal substrates for epitaxial deposition of thin film Ba--Y--Cu--O type superconductors which provide sufficiently close lattice match between substrate and film.